An electromagnetic clutch including a driving pulley and a shaft being coaxially arranged and a wrap spring in a coil form having a center at a rotation axis, which connects and disconnects a motive power by changing a winding diameter of the wrap spring, is known. In JPH11-201191A, hereinafter referred to as Reference 1, an electromagnetic clutch similar to the known electromagnetic clutch is disclosed. In the electromagnetic clutch disclosed in Reference 1, an input hub and an output hub in rotatable states arranged coaxial to a rotation axis are provided at adjacent positions to each other along the rotation axis. A wrap spring is arranged at an outer periphery of the input hub and an outer periphery of the output hub. A control collar is arranged around the wrap spring. One end of the wrap spring is retained at the input hub and the other end of the wrap spring is retained at the control collar.
According to the electromagnetic clutch disclosed in Reference 1, the clutch is disconnected while a magnetic force is not acting on an armature provided on the control collar. In the state where the clutch is disconnected, the armature is retained at a predetermined position by a biasing force of a return spring so that the wrap spring is detached from the outer periphery of the input hub and the outer periphery of the output hub. In the state where the magnetic force is acting on the armature provided on the control collar, the magnetic force attracts the armature and pulls the armature in a direction of a magnetic solenoid against the biasing force of the return spring so that a rotational force of the control collar is transmitted to the output hub. Accordingly, the wrap spring is brought to a state where the winding diameter of the wrap spring is reduced which in turn results in the wrap spring in a pressure bonding state relative to the outer periphery of the input hub and the outer periphery of the output hub and provides a state where the electromagnetic clutch is connected, which is a state where the rotational driving power from the input hub is transmitted to the output hub.
In JP2001-317565A, hereinafter referred to as Reference 2, an electromagnetic clutch including a wrap spring provided over a span of and arranged around an outer periphery of a driving pulley and an outer periphery of an output hub that integrally rotates with a shaft is disclosed. The driving pulley is referred to as an input pulley and the shaft is referred to as a pump shaft in Reference 2. One end of the wrap spring is retained at an armature and the other end of the wrap spring is retained at the output hub. An electromagnetic solenoid, which provides an attraction force acting on the armature, is arranged at an inwardly position relative to the driving pulley.
In a state where a magnetic force from the electromagnetic solenoid is not acting on the armature, the electromagnetic clutch disclosed in Reference 2 is in a disconnected state, which is a state where the wrap spring is detached from the outer periphery of the input pulley and the outer periphery of the output hub. In a state where the electromagnetic solenoid is activated, the magnetic force from the electromagnetic solenoid attracts the armature toward the electromagnetic solenoid, which in turn results in attaching the wrap spring firmly over the span of the outer periphery of the input pulley and the outer periphery of the output hub and provides a state where the electromagnetic clutch is connected, which is a state where the driving power from the driving pulley is transmitted to the shaft.
As Reference 1 and Reference 2 describe, in the electromagnetic clutches providing a power transmission state initiated by supplying power to the electromagnetic solenoid, a continuous electric power supply is required in order to transmit motive power. As a result, excessive electricity is consumed by the electromagnetic clutch in a situation where the electromagnetic clutch is applied to a portion, for example, to a water pump for an engine, where the power transmission state, or the state where the electromagnetic clutch is in the connected state, continues for a long period of time. The electromagnetic clutch that maintains the power transmission state by continuously supplying electricity requires a large size electromagnetic solenoid for providing adequate magnetic flux in order to maintain a reliable power transmission state even in a state where the magnetic flux decreases when the temperature rises. Using the large size electromagnetic solenoid leads to a cost increase. The water pump provided with the electromagnetic clutch does not function appropriately when the power supply is discontinued due to a malfunctioning, which leads to an overheating of the engine.
The arrangement of the electromagnetic clutch described in Reference 1 is formed with an air gap at a portion in a magnetic circuit formed between the electromagnetic solenoid and the armature, which more specifically is at a portion between a rotor that rotates by connecting to the output hub and the electromagnetic coil in a fixed state. The air gap leads to an increase in an amount of electricity the electromagnetic solenoid requires. Furthermore, the input hub and the output hub are arranged adjacently to each other in the electromagnetic clutch described in Reference 1. Accordingly, the input hub and the output hub may unintentionally rotate together even in a state where the electromagnetic solenoid is not activated depending on an attitude of the wrap spring or a condition of a contact state between the wrap spring and the output hub.
Similarly, the arrangement of the electromagnetic clutch described in Reference 2 may require a powerful electromagnetic solenoid because of an air gap available even in a connected state of the electromagnetic clutch at a portion in a magnetic circuit formed between the electromagnetic solenoid and the armature, which more specifically is at a portion between the input pulley in a rotating state and the electromagnetic solenoid in a fixed state.
Switching swiftly between the connected state and the disconnected state is favorable in an electromagnetic clutch that connects and disconnects a driving power from a driving-side rotation member to a driven-side rotation member.
A need thus exists for an electromagnetic clutch, which is not susceptible to the drawback mentioned above.